I think I got a batch of batteries that don't live up to their capacity (what a surprise!), so how do I check capacity using a DMM? I adjusted the nob to CAP, but it blinks and gives me 0000. The batteries have voltage, but how do I determine their capacity?
The sad thing is, of this massive resource known as the internet, I could not find a single solitary tutorial or video that shows me how, just car battery demos!
You can't with a DMM, you need a hobby charger
Your best bet would be to just measure runtime in a torch you have. Check the tailcap current and obviously terminate at just under 3v or use a torch with known good low voltage cut-off, tho by 3v it will be dim and drawing much less current. This won't give you a figure but can be compared and guessed?
Edit, I'd make sure you put a fan on the torch to keep it cool also.
+1
Measure the current every 5 minutes as the batteries discharge. Figure an average value for the current in amps and multiply by the runtime in hours. This will give a value in amphours at the current the light was set to. Lower current settings will give higher Ah totals, especially with older or inferior batteries with high parasitic resistance. Not difficult and can be done with Dmm. Use short thick leads to the meter to get more accurate tailcap readings.